Reporting of channel state information

ABSTRACT

A mobile station is connected to a multi-carrier cellular communication system having a plurality of sub-carriers. The sub-carriers are classified into K frequency sub-bands, channel state information (CSI) of the frequency sub-bands is represented by matrices Wi (i=0 . . . K−1), where K is an integer grater than 1. The mobile station determines a first sub-index k1 for the K matrices Wi (i=0 . . . K−1), and a second sub-index k2 for each one of the K matrices Wi (i=0 . . . K−1). The first sub-index k1 is common for all frequency sub-bands, and the second sub-index k2 is specific for the indexed matrix that corresponds to one frequency sub-band. The mobile station reports to a base station of the multi-carrier cellular communication system the first sub-index k1 and at least one second sub-index k2.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/621,350, filed on Feb. 12, 2015, now U.S. Pat. No. 9,876,550. TheU.S. patent application Ser. No. 14/621,350 is a continuation of U.S.patent application Ser. No. 13/784,312, filed on Mar. 4, 2013, now U.S.Pat. No. 8,983,001, which is a continuation of U.S. patent applicationSer. No. 13/544,878, filed on Jul. 9, 2012, now U.S. Pat. No. 8,837,607.The U.S. patent application Ser. No. 13/544,878 is a continuation ofInternational Application No. PCT/CN2010/079938, filed on Dec. 17, 2010.The International Application No. PCT/CN2010/079938 claims priorities toInternational Application No. PCT/SE2010/000002 filed on Jan. 8, 2010,and Swedish Patent Application No. SE1000015-6, filed on Jan. 8, 2010.All of the aforementioned patent applications are hereby incorporated byreference in their entireties.

TECHNICAL FIELD

The present invention relates to a method and apparatus of reportingchannel state information in wireless communication. In particularembodiments, it relates to a method and apparatus of such informationreporting of channel state(s) having a codebook representative.

BACKGROUND

It is well known in the art that the performance is greatly enhanced iflinear precoding can be used at a transmitter side in wirelesscommunication systems supporting multiple antenna transmissions. Suchlinear precoding has been implemented, e.g., in the IEEE 802.16-2005standard and in the 3GPP Rel.8 Long Term Evolution (LTE) standard.

To support precoding at the transmitter side, the receiver, sometimesalso known as a User Equipment (UE) in the Downlink (DL), needs to feedback Channel State Information (CSI) about the multi-antenna channelbetween transmit and receive antennas. The CSI may consist of arepresentation of the actual multi-antenna channel, or alternatively apreferred precoding vector/matrix which the UE has determined based onmeasurements on the multi-antenna channel. In the latter case, the CSIis commonly referred to as a Precoding Matrix Indicator (PMI).

To reduce feedback overhead when signalling CSI reports, quantization isrequired so as to represent the CSI in a finite number of bits. As anexample, the 3GPP LTE Rel.8 standard use a precoding matrix codebookconsisting of 64 matrices and the UE feeds back the preferred precodingmatrix using six information bits.

As mentioned above, a codebook of a finite number of matrices iscommonly used to quantize the CSI in which case the CSI feedback is anindex to a codebook that points out the matrix in the codebook that bestrepresents the CSI. The index is then reported to the transmit nodeusing for instance a string of binary bits.

As the channel is frequency and time selective by nature, a CSI reportis only valid with reasonable accuracy up to some maximum bandwidth andfor some maximum time. If the communication system wants to supporttransmission bandwidths using linear precoding for larger than thismaximum bandwidth, feedback of multiple CSI reports is needed andfurther these CSI reports need to be repeated in time with appropriateintervals.

The bandwidth and time interval of each of these CSI reports are denotedas time-frequency granularity of the CSI, and if a codebook of matricesis used to quantize the CSI, one matrix is reported per time intervaland frequency bandwidth.

To meet the high requirements on data throughput in future wirelesscommunication systems, such as the 3GPP LTE-Advanced, an even largernumber of transmitter and receiver antennas are envisioned. Since thedimensions of the multi-antenna channel thereby increases, the requiredCSI feedback overhead will increase further, thereby hampering thedesired throughput increase.

Furthermore, when the number of antennas (or antenna elements) isincreased, the physical dimensions of the transmitter and receivers willalso increase, which is undesirable due to the larger area of, e.g., aBase Station (BS) which will make it more vulnerable to environmentaleffects such as strong winds. Also, the architectural (visible) impacton buildings and the effect on landscape or cityscape should not beneglected in this context. To partly cope with the problem of largerantenna arrays dual polarized antenna elements is commonly assumed,since by utilizing two orthogonal polarizations of the electromagneticfield, one can effectively have two antennas in one. So, by utilizingpolarized antennas, the total dimension of the antenna arrays is roughlyhalved.

Another obvious approach to make equipment with many antenna elementsphysically smaller is to reduce the spacing between the antennaelements. This will make signals received and transmitted morecorrelated (if they have the same polarization) and it is well knownthat the expected multiple antenna spatial multiplexing gain will bereduced. However, it is also known that correlated signals make verygood and narrow beams, and the multiple antenna spatial multiplexingcould then be used to transmit to users which are spatially separated.This is sometimes called Spatial Division Multiple Access (SDMA) orMulti-User MIMO (MU-MIMO). Hence, the drawback of lower per userthroughput when using narrow spaced antennas elements can be compensatedby transmitting to multiple users simultaneously, which will increasethe total cell throughput (i.e., the sum of all users throughput in thecell).

Further, it is a known physical property that the channel emanating fromantennas with orthogonal polarizations have close to independent fading.It is further known that channels emanating from closely spaced equallypolarized antenna elements have correlated fading. Hence, formulti-antenna transmitters and receivers having a large number ofantenna elements, compact antenna arrays which also utilize thepolarization dimension is preferred. In this case, it is observed thatamong the antenna elements, the correlation between the radio channelsbetween some pairs of the antenna elements is high, whereas thecorrelation for the radio channels between some other pairs of theantenna elements is low or even negligible. It is often said that twoantennas are correlated meaning that the channel from the two antennasto any receiver antenna are correlated. This convention is usedthroughout the present disclosure.

In the 3GPP LTE standard specification TS 36.211, a codebook of 16matrices is defined which facilitates feedback of dual polarized antennaarrays. Each matrix is thus indexed with a single 4 bit index. Thefeedback can be per sub-band which is a limited part of the totalavailable bandwidth, or wideband which is the whole available bandwidth,i.e. the sum of all sub-bands. Hence, according to said specification a4 bit PMI is fed back for each of N number of sub-bands, or for thewideband case. Therefore, 4*N feedback bits is needed when using themethod in the TS 36.211 specification.

SUMMARY

An aspect of the present invention is to provide a method for reportingCSI with reduced overhead compared to prior art solutions. Also anaspect of the invention is to provide a method of reporting CSI withimproved accuracy.

Example embodiments of the invention provide for channel stateinformation (CSI) reporting by means of a codebook representationcomprising a number of one or more matrices arranged according to two ormore sub-indices between a transmit node (TN) and a receive node (RN) ina wireless communication system being arranged for providing orprocessing the sub-indices with index-specific time-frequency reportinggranularity as further explained in the subsequent detailed description.

Thereby, CSI reporting overhead may reduce and facilitate increaseduplink data throughput. CSI reporting accuracy may also improve and,e.g., result in increased downlink user throughput and increaseddownlink cell throughput.

Furthermore, required receiver complexity is reduced according topreferred embodiments, wherein the first sub-index is common to allsub-bands, and the selection of the first sub-index need not be repeatedfor every sub-band. Yet another benefit of an example embodiment of theinvention is that the codebook may be structured in a suitable way fortaking advantage of and transferring CSI reports according tocharacteristics of commonly used dual polarized antenna arrays.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawing, in which:

FIG. 1 is an example of a dual polarized antenna array with six antennaelements V1, V2, V3, H1, H2 and H3, wherein the elements V1, V2 and V3have the same polarization, which is orthogonal to the polarization ofantenna elements H1, H2 and H3;

FIG. 2 is an example where a full bandwidth is divided into foursub-bands, and wherein for each sub-band a preferred precoding matrix isselected and reported. The first sub-index is the same for all sub-bandsin this example, but the second sub-index may be different for eachsub-band;

FIG. 3 is a flow chart of how a receive node may select a matrix indexper sub-band and transmit a CSI report;

FIG. 4 is a flow chart of how a CSI per sub-band may be reconstructed bya transmit node of a CSI report received from a receive node;

FIG. 5 shows an example of the use of wideband CSI feedback reports andper sub-band CSI feedback reports;

FIG. 6 shows an example of transmission between a transmit node (TN) anda receive node (RN) in a wireless communication system;

FIG. 7 shows time-frequency representation of a channel in amulti-carrier communication system, wherein each rectangle representsthe time-frequency granularity by frequency bandwidth F and timeduration T;

FIG. 8 shows a block diagram of a particular example of a RN, which is amobile station having a processor and a transceiver for reportingchannel state information in a multi-carrier cellular communicationsystem; and

FIG. 9 shows a block diagram of another particular example of a RN,which is a mobile station having processing circuitries for reportingchannel state information in a multi-carrier cellular communicationsystem.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In communications relying upon, channel state information, CSI,retrieved in circuitry adapted therefore and reported/fed back from areceive node (RN) to a transmit node (TN) in a wireless communicationsystem as shown in FIG. 6, the transmit node preferably comprisesprocessing circuitry adapted for applying this information about thestate of the radio channel to improve the subsequent multiple antennatransmission of data to the receive node.

In a multi-carrier communication system, such as LTE or LTE-A, theunderlying channel for a given subcarrier between a transmit node and areceive node is preferably represented by a N_(r) times N_(t) matrix W,where N_(t) is the number of transmit antennas and N_(r) is the numberof receive antennas, respectively. The CSI feedback can therefore be aquantization of the N_(r) times N_(t) matrix W using a codebook C ofmatrices, or W can also be a preferred precoding matrix having sizeN_(t) times R, where 1≤R≤min(N_(r), N_(t)) is the preferred transmissionrank. The transmission rank, also known as the number of layers,determines the degree of spatial multiplexing in a transmission.

Since the radio channel is fading in the frequency and time domain, theselected matrix or equivalently the CSI will only be valid for somefrequency bandwidth F and time interval T, which henceforth will bedenoted as the time-frequency granularity of a CSI feedback report.

FIG. 7 illustrates time-frequency granularity for a multicarriercommunication system, wherein each rectangle represents a frequencybandwidth and a time duration for which the CSI can be consideredconstant or similar, and therefore one CSI report is required for eachof the rectangles in FIG. 7 to accurately describe the CSI for the wholetransmission bandwidth during the time interval T.

So, for reporting the CSI for a bandwidth that is K times larger than Fa CSI report will consist of K number of CSIs, where each of the CSIreports reflects the channel state in a corresponding frequencybandwidth F and time interval T. A feedback report thus contains Knumber of matrices W^(i) from a codebook C, where K≥1 and i=0 . . . K−1.

It has further been observed by the inventors that correlated antennasor the channel from a group of correlated antennas have a largercoherence bandwidth of the spatial correlation and longer coherence timecompared to antennas, or between antenna groups with lower correlation,and could therefore have a lower granularity in its feedback. Therefore,even though a CSI feedback matrix W^(i) has a time-frequency granularitywith frequency bandwidth F and time interval T, it is possible tostructure the matrix W^(i) in such way that different sub-parts of thismatrix structure has a lower time-frequency granularity, i.e., F′ and/orT′, or equivalently F′>F and/or T′>T. Different structures of a CSImatrix W^(i) are covered by different embodiments of the presentinvention.

The exploitation of the antenna correlation in a CSI report is achievedby introducing a special structure of the matrices belonging to thecodebook C, which allows for indexing the matrices in the codebook C byP number of sub-indices, where P>1. Also, by using multiple sub-indicesaccording to the invention, each one of the sub-indices may have its owntime-frequency granularity. This is an effective and low complex way ofreducing the total number of bits required to report the CSI andtherefore advantage over prior art.

Hence, the present invention relates to a method for reporting CSIbetween a transmit node and a receive node in a wireless communicationsystem. The transmit node being arranged for multiple transmit antennatransmissions and the CSI being represented by K number of matricesW^(i) from a codebook C comprising a plurality of matrices, wherein eachmatrix in the codebook C represents a state, or a preferred precodingmatrix, for the multi-antenna channel from the transmit node to thereceive node, K≥1 and i=0 . . . K−1. The method comprises the steps of:reporting a first sub-index k₁; and reporting at least one additionalsecond sub-index k₂ for each one of the K number of matrices W^(i),wherein each one of the K number of matrices W^(i) is indexed by thefirst k and second k₂ sub-indices, and the first k₁ and second k₂sub-indices have different time-frequency reporting granularity.

The invention can therefore be understood as that the first sub-index k₁indicates a matrix M_(k) ₁ from a first sub-codebook C₁ and that the atleast one additional second sub-index k₂ indicates a matrix M_(k) ₂ ^(i)from a second sub-codebook C₂ so that each one of the K number ofmatrices W^(i) in codebook C are generated from the first M_(k) ₁ andsecond matrices M_(k) ₂ ^(i), which are indexed by the first k₁ and atleast one additional second k₂ sub-index, respectively.

Therefore, the present invention can reduce CSI feedback overhead, oralternatively by keeping the same feedback overhead, it is possible toimprove CSI feedback reporting accuracy since each feedback bits will bemore efficiently utilized. This is achieved by taking advantage of thecorrelations of the radio channel, which is the same for common antennasetups in the art, such as a narrow spaced antenna element array withdual polarized antenna elements.

As an example of an implementation of the invention: assume a codebook Cwith 32 matrices, so 5 bits is required to index a matrix W^(i)belonging to said codebook C. If the matrix W^(i) is indexed by at leasttwo sub-indices k₁, k₂, where a first sub-index k₁ is a three bit indexand a second sub-index k₂ is a two bit index. Hence, to represent amatrix, both sub-indices k₁, k₂ are needed and in total 5 bits are used.As mentioned, the different sub-indices k₁ and k₂ are fed back withdifferent granularity in time and/or frequency. This has feedbackoverhead benefits. For instance, the first sub-index k₁ can be reportedevery 10 milliseconds whereas the second sub-index k₂ is reported every1 millisecond. The receiver of the feedback message thus updates thefirst sub-index k₁ in the index pair k₁, k₂ less frequently than thesecond sub-index k₂. The overhead benefits of this is arrangement isobvious; instead of feeding back 5 bits per millisecond or 5 kbit/s, theoverhead has been reduced to effectively (3+2*10)/10=2.3 bits permillisecond or equivalently 2.3 kbit/s.

The same principle can be applied for reducing overhead when reportingCSI for multiple sub-bands in frequency, where the selection of thefirst sub-index k₁ can be valid for a bandwidth of 300 subcarriers, andwhereas the selection of the second sub-index k₂ is valid for a sub-bandbandwidth of 30 subcarriers. Hence, if the total bandwidth is 300subcarriers, one first sub-index k₁ and ten second sub-indices k₂ mustbe reported to indicate the matrix W^(i) of each sub-band. The feedbackoverhead to support per sub-band CSI feedback is thus reduced from 50bits (if 5 bits is reported for each of the 10 sub-bands) to 23 bits (if3 bits is used for the first sub-index k₁ and 2 bits for each of thesecond sub-indices k₂).

The principle can also be used to improve the accuracy of the CSIreport, where the selection of the first sub-index k₁ can be valid for abandwidth of 300 subcarriers, whereas the selection of the secondsub-index k₂ is valid for a sub-band bandwidth of 15 subcarriers, whichis less than the 30 subcarriers in the previous example. Due to thereduced sub-bandwidth, each CSI report will represent the CSI for asmall frequency bandwidth and therefore be better matched to the radiochannel, i.e., have better accuracy. Hence, assuming that the totalbandwidth includes 300 subcarriers, one first sub-index k₁ and twentysecond sub-indices k must be reported to indicate the matrix W^(i) foreach sub-band. The feedback overhead to support per sub-band CSIfeedback is thus 43 bits (if 3 bits is used for the first sub-index k₁and 2 bits for each of the second sub-indices k₂).

An alternative way to improve the accuracy of the CSI report is toincrease the codebook size. Assuming that the selection of the firstsub-index k₁ is valid for a bandwidth of 300 subcarriers, whereas theselection of the second sub-index k₂ is valid for a sub-band bandwidthof 30 subcarriers. Hence, if the total bandwidth is 300 subcarriers, onefirst sub-index k₁ and ten second sub-indices k₂ must be reported toindicate the matrix W^(i) for each sub-band. The feedback overhead tosupport per sub-band CSI feedback is again 43 bits but with a largercodebook (if 3 bits is used for the first sub-index k₁ and 4 bits foreach of the sub-indices k₂).

A further example of per sub-band CSI feedback is given in FIG. 2 wherethe full bandwidth is divided into four sub-bands and a matrix from acodebook with P=2 has been used, where P denotes the number ofsub-indices used to index each matrix W^(i) in the codebook C. The firstsub-index for each matrix W^(i) is common to all sub-bands and needsthus only be reported once, whereas the second sub-index is dependent onthe sub-band number.

In FIG. 1, a dual polarized antenna array with six antenna elements isillustrated. Each of the two polarizations creates a linear antennaarray with three antenna elements. The antenna elements with the samepolarization (e.g., V1 and V2) will have a high correlation (i.e., willgenerate channels with high correlation) if they are placed with narrowspacing, but the correlation between antenna elements with differentcorrelation (e.g., V1 and H1) will generally have a low or a negligiblecorrelation. This dual polarized antenna array structure is a preferredantenna setup when many antenna elements at the transmitter is desirabledue to the compact size obtained by utilizing two polarization antennas.The size of the antenna array can be reduced further by selecting asmall separation between antenna elements having the same polarization.

Therefore, according to an embodiment of the invention aforementionedcorrelation properties of dual polarized antenna arrays is takenadvantage of as it is known that closely spaced antennas with the samepolarization have high correlation whereas antennas with orthogonalpolarizations have low correlation. If a first group of antenna elementsare numbered 1 to N_(t)/2 and are arranged so that they have the samepolarization and a second group of antenna elements are numberedN_(t)/2+1 to N_(t) have the same but orthogonal polarization withrespect to the first group. Then a matrix codebook C with P=2 indicescan be structured as:

$\begin{matrix}{{W^{i} = {{\begin{bmatrix}M_{k_{1}} \\{M_{k_{1}}M_{k_{2}}^{i}}\end{bmatrix}\mspace{14mu}{or}\mspace{14mu} W^{i}} = \begin{bmatrix}M_{k_{1}} \\{M_{k_{2}}^{i}M_{k_{1}}}\end{bmatrix}}},} & (1)\end{matrix}$where the matrices M_(k) ₁ and M_(k) ₂ ^(i) are taken from a first and asecond sub-codebook C₁ and C₂, indexed by the first and secondsub-matrices, respectively. The matrix M_(k) ₁ then reflects theprecoding matrix for correlated antenna elements and can then beoptimized for this scenario using, e.g., Discrete Fourier Transform(DFT) matrices. The matrix M_(k) ₂ ^(i), which may be a diagonal matrix,then reflects the phase and the amplitude relationship between the twopolarizations. The matrix W^(i) is thus indexed by two sub-indices k₁,k₂, where the first sub-index k₁ can be reported back with lowergranularity in frequency and/or time, since this sub-index correspondsto the correlated subset of antenna elements, whereas the secondsub-index k₂ can be reported back with higher granularity in frequencyand/or time (e.g., higher sampling rate in frequency and/or time), sincethe second sub-index corresponds to the relation between the twodifferent polarization directions.

In another embodiment of the invention a matrix W^(i) from the codebookC derived by two matrices from sub-codebooks C₁ and C₂ by equation (1)can also be written as:

$\begin{matrix}{{W^{i} = {\begin{bmatrix}M_{k_{1}} \\{M_{k_{1}}D_{k_{2}}}\end{bmatrix} = {\underset{\underset{A_{k_{1}}}{︸}}{\begin{pmatrix}M_{k_{1}} & 0 \\0 & M_{k_{1}}\end{pmatrix}}\underset{\underset{B_{k_{2}}}{︸}}{\begin{pmatrix}I \\D_{k_{2}}\end{pmatrix}}}}},} & (2)\end{matrix}$where a product of two matrices, each with different indices describesthe underlying matrix structure. Hence, in equation (2) differenttime-frequency granularity is used for reporting the index to the twomatrices M_(k) ₁ and M_(k) ₂ ^(i), respectively.

In a yet another embodiment of the invention matrices W^(i) in codebookC with P=2 sub-indices can be structured as:W ^(i) =M _(k) ₂ ^(i) ⊗M _(k) ₁ or W ^(i) =M _(k) ₁ ⊗M _(k) ₂ ^(i)  (3),where ⊗ is the Kronecker product and the matrices M_(k) ₁ and M_(k) ₂^(i) are obtained from the sub-codebooks C₁ and C₂, respectively. Thetime-frequency granularity of the reporting of matrix M_(k) ₁ may behigher or lower than for matrix M_(k) ₂ ^(i).

In a yet another embodiment matrices W^(i) in codebook C with P=2sub-indices can be structured as:W ^(i) =M _(k) ₂ ^(i) M _(k) ₁ or W ^(i) =M _(k) ₁ M _(k) ₂ ^(i)  (4),which is a general structure where one of the matrices is the widebandrepresentation and the other matrix is the feed back per sub-bandrepresentation.

In a further embodiment of the invention matrices W^(i) in the codebookC are structured as:

$\begin{matrix}{{W^{i} = \begin{bmatrix}M_{k_{1}} \\{M_{k_{1}}e^{{jd}_{k_{2}}}}\end{bmatrix}},} & (5)\end{matrix}$wherein d_(k) ₂ is a scalar.

In a further embodiment, the number of sub-indices is P=3 and matricesW^(i) in the codebook C are indexed by three sub-indices (k₁, k₂ and k₃,respectively) and can be structured as:

$\begin{matrix}{{W^{i} = {{\begin{bmatrix}M_{k_{1}} \\{M_{k_{2}}^{i}M_{k_{3}}}\end{bmatrix}\mspace{14mu}{or}\mspace{14mu} W^{i}} = \begin{bmatrix}M_{k_{1}} \\{M_{k_{3}}M_{k_{2}}^{i}}\end{bmatrix}}},} & (6)\end{matrix}$wherein M_(k) ₁ and M_(k) ₃ reflects precoding matrices for correlatedantennas so k₁, k₃ are reported back with lower time-frequencygranularity than the second sub-index k₂ for the matrix M_(k) ₂ ^(i)which reflects the relationship between the polarizations and is thusfeed back with higher granularity in frequency and/or time.

According to the 3GPP LTE and LTE-Advanced standards, there exist twofeedback reporting possibilities, i.e. using the Physical Uplink ControlChannel (PUCCH) and the Physical Uplink Shared Channel (PUSCH). ThePUCCH is configured to transmit few bits (low payload) periodically,whilst the PUSCH can carry a large number of bits and is a scheduledaperiodic resource. Furthermore, a useful property of CSI feedback isthat each report is self-contained meaning that one should not have torely on multiple reports fed back at different time instants to computethe CSI for a given sub-band. Therefore, the PUCCH is suitable forwideband feedback meaning that the PUCCH contain some average CSIinformation for the whole feedback bandwidth. The PUSCH on the otherhand which has less limitation on the payload, and thus can carry morebits, is better suited for the detailed per sub-band CSI. An example ofthis feedback structure is shown in FIG. 5. In this figure the height ofthe bars, at each reporting instant, illustrates the number of bitsreported in that CSI report. The PUCCH report is periodic, whereas thePUSCH report may be aperiodic meaning that the transmitting noderequests a PUSCH report when needed. It is however also possible toconfigure a periodic PUSCH report with some other periodicity than thePUCCH report, which is well understood by the skilled person in the art.

The use of the PUCCH for wideband feedback can be supported by thepresent invention by feeding back the P number of sub-indices and wherethere is only one sub-band spanning the whole bandwidth reflecting awideband CSI in the PUCCH. Furthermore, overhead can be reduced furtherby introducing some relationship among the P indices. This can beexplained by an example. Assume the matrix structure:

$\begin{matrix}{{W^{i} = \begin{bmatrix}M_{k_{1}} \\{M_{k_{1}}e^{{jd}_{k_{2}}}}\end{bmatrix}},} & (7)\end{matrix}$where the first sub-codebook C₁ for the matrix M_(k) ₁ is given by:

$\begin{matrix}{{C_{1} = \left\{ {\begin{bmatrix}1 \\1\end{bmatrix},\begin{bmatrix}1 \\{- 1}\end{bmatrix},\begin{bmatrix}1 \\i\end{bmatrix},\begin{bmatrix}1 \\{- i}\end{bmatrix}} \right\}},} & (8)\end{matrix}$where i=√{square root over (−1)}, and the second sub-codebook C₂contains scalars d_(k) ₂ given by:C ₂={π−π}  (9).

Hence, the first sub-index k₁ is in this example represented by two bitswhereas the second sub-index k₂ is represented by one bit only. Assumingthat there are 10 sub-bands in a per sub-band feedback suitable for thePUSCH, then 2+10*1=12 bits are reported back. In a wideband feedbacksuitable for the PUCCH, 2+1=3 bits are fed back.

As mentioned above, it is possible to further reduce the feedbackoverhead to two bits by introducing dependencies between the P number ofsub-indices. In a feedback container, in which the payload is verylimited, it is possible to make a table of “allowed” pairs of Psub-indices. An example of this is shown in Table 1 below. Although thefull codebook contains 8 elements and requires three bits, this has beenreduced to two bits in the example in Table 1. To select which matricesthat should be included in the table, it is possible to resort todifferent optimization methods, and for instance aim at maximizing theminimum Chordal distance between the remaining matrices.

TABLE 1 Example on how to reduce feedback overhead by introducingdependencies between codebook indices k₁ k₂ $W^{i} = \begin{bmatrix}M_{k_{1}} \\{M_{k_{1}}e^{{jd}_{k_{2}}}}\end{bmatrix}$ 0 0 $W^{i} = \begin{bmatrix}1 \\1 \\1 \\1\end{bmatrix}$ 1 1 $W^{i} = \begin{bmatrix}1 \\{- 1} \\{- 1} \\1\end{bmatrix}$ 2 0 $W^{i} = \begin{bmatrix}1 \\i \\1 \\i\end{bmatrix}$ 3 1 $W^{i} = \begin{bmatrix}1 \\{- i} \\{- 1} \\i\end{bmatrix}$

It should also be noted that the matrices W^(i) in the codebook C mayused as PMIs, which is obvious to the skilled person.

Furthermore, the invention also relates to a method in a receive node, amethod in a transmit node and devices thereof.

One embodiment method can be used in a transmit node for receiving andprocessing CSI between the transmit node and a receive node, where theCSI being represented by K number of matrices W^(i) from a codebook Ccomprises a plurality of matrices. The method includes receiving a firstsub-index k₁ and receiving at least one additional second sub-index k₂for each one of the K number of matrices W^(i). Each one of the K numberof matrices W^(i) which are indexed by said first k₁ and second k₂sub-indices are reconstructed. The first k₁ and second k₂ sub-indiceshave different time-frequency reporting granularity. A transmit nodedevice thereof is also disclosed in the present application.

Another embodiment method can be used in a receive node for reportingCSI between a transmit node and the receive node. The CSI beingrepresented by K number of matrices W^(i) from a codebook C comprises aplurality of matrices. In the embodiment method, the receive nodereports a first sub-index k₁ and at least one additional secondsub-index k₂ for each one of the K number of matrices W^(i). Each one ofthe K number of matrices W^(i) is indexed by the first k₁ and second k₁sub-indices, and the first k₁ and second k₂ sub-indices have differenttime-frequency reporting granularity. A receive node device thereof isalso disclosed in the present application.

It is also understood by the skilled person that the method in atransmit node and the method in a receive node may be modified, mutatismutandis, according to the different embodiments of the method in awireless communication system according to the present invention.

Moreover, an example flowchart of how a receive node, which reports theCSI to a transmit node, selects the matrix per each of the K number ofsub-bands (i.e. sub-indices k₁, k₂ per sub-band in this particularexample) and report back this information is given in FIG. 3.

Block 301: Computing k₁ as common for all sub-bands;

Block 303: Computing k₂ individually for each sub-band, conditioned onselected k₁;

Block 305: Reporting (transmitting) one k₁ and at least one k₂ persub-band, effectively one k₁, k₂ index pair per sub-band, wherein k₁ andk₂ have different time-frequency reporting granularity.

In FIG. 4, an example flowchart is given of how a transmit nodereconstructs the CSI for each of the K number of sub-bands.

Block 401: Receiving one k₁ and at least one additional k₂;

Block 403: For each sub-band: generating the index pair k₁, k₂ using thefirst sub-index k₁ and sub-band dependent second sub-index k₂;

Block 405: Use the index pair k₁, k₂ for each of K sub-bands to find thecorresponding matrix W^(i), i=0 . . . K−1 in the codebook C by usingfirst and second sub-codebooks.

In FIG. 4, The transmit node has received one first sub-index k₁ and Knumber of second sub-indices k₂ (one for each sub-band) and uses thesub-indices and the first and second sub-codebooks C₁ and C₂ forreconstructing the matrix W^(i) for the i:th sub-band. The matrix W^(i)hence represents the CSI for sub-band i and is a matrix in the codebookC.

Furthermore, as understood by the person skilled in the art, the methodin a receive node and the method in a transmit node according to thepresent invention may be implemented in a computer program, having codemeans, which when run in a computer causes the computer to execute thesteps of the method. As particular examples, FIG. 8 illustrates a RNwhich is a mobile station having a transceiver and a processor forreporting channel state information, and FIG. 9 illustrates a RN whichis a mobile station having processing circuitries in an apparatus forreporting channel state information. The computer program is included ina computer readable medium of a computer program product. The computerreadable medium may consist of essentially any memory, such as a ROM(Read-Only Memory), a PROM (Programmable Read-Only Memory), an EPROM(Erasable PROM), a Flash memory, an EEPROM (Electrically Erasable PROM),or a hard disk drive.

It should finally be understood that the present invention is notlimited to the embodiments described above, but also relates to andincorporates all embodiments within the scope of the appendedindependent claims.

What is claimed is:
 1. A non-transitory computer-readable storage mediumcomprising instructions which, when executed by a computer, cause thecomputer to carry out the steps of: determining K matrices for Kfrequency sub-bands, wherein K precoding matrix indicators (PMIs) of theK frequency sub-bands respectively represent the K matrices, a pluralityof sub-carriers in a multi-carrier cellular communication system isdivided into the K frequency sub-bands, and K is an integer greater than1; determining a first sub-index and K second sub-indices, wherein eachof the K matrices is indexed by the first sub-index and one of the Ksecond sub-indices, and wherein each of K PMIs comprises the firstsub-index and one of the K second sub-indices; and transmitting, to abase station, the first sub-index and at least one of the K secondsub-indices, wherein the first sub-index is common for all of the Kfrequency sub-bands, and each of the K second sub-indices is specificfor a matrix that corresponds to one frequency sub-band of the Kfrequency sub-bands.
 2. The non-transitory computer-readable storagemedium according to claim 1, wherein the first sub-index indicates afirst sub-matrix from a first sub-codebook, and a second sub-index inthe K second sub-indices indicates a second sub-matrix from a secondsub-codebook, wherein each one of the K matrices comprises the firstsub-matrix and one of the K matrices comprises the second sub-matrix. 3.The non-transitory computer-readable storage medium according to claim2, wherein each one of the K matrices is represented as W^(i), 0≤i≤K−1,and ${W^{i} = \begin{bmatrix}M_{k_{1}} \\{M_{k_{1}}M_{k_{2}}^{i}}\end{bmatrix}},$ where W^(i) represents the i^(th) matrix, k₁ representsa first sub-index identifying a first sub-matrix M_(k) _(i) , and k₂represents a second sub-index identifying a second sub-matrix M_(k) ₂^(i).
 4. The non-transitory computer-readable storage medium accordingto claim 2, wherein each one of the K matrices is represented as W^(i),0≤i≤K−1, and ${W^{i} = \begin{bmatrix}M_{k_{1}} \\{M_{k_{2}}^{i}M_{k_{1}}}\end{bmatrix}},$ where W^(i) represents the i^(th) matrix, k₁ representsa first sub-index identifying a first sub-matrix M_(k) _(i) , k₂represents a second sub-index identifying a second sub-matrix M_(k) ₂^(i).
 5. The non-transitory computer-readable storage medium accordingto claim 2, wherein each one of the K matrices is represented as W^(i),0≤i≤K−1, andW ^(i) =M _(k) ₂ ^(i) M _(k) ₁ or W ^(i) =M _(k) ₁ M _(k) ₂ ^(i), whereW^(i) represents the i^(th) matrix, k₁ represents a first sub-indexidentifying a first sub-matrix M_(k) ₁ , k₂ represents a secondsub-index identifying second sub-matrix M_(k) ₂ ^(i).
 6. Thenon-transitory computer-readable storage medium according to claim 2,wherein each one of the K matrices is represented as W^(i), 0≤i≤K−1, and${W^{i} = \begin{bmatrix}M_{k_{1}} \\{M_{k_{1}}e^{{jd}_{k_{2}}}}\end{bmatrix}},$ where W^(i) represents the i^(th) matrix, k₁ representsa first sub-index identifying a first sub-matrix M_(k) ₁ , k₂ representsa second sub-index identifying d_(k) ₂ , and d_(k) ₂ represents ascalar.
 7. The non-transitory computer-readable storage medium accordingto claim 1, wherein the multi-carrier cellular communication system is along term evolution (LTE) system or a long term evolution advanced(LTE-A) system.
 8. An apparatus in a mobile station, comprising: amemory storing executable instructions; and a processor coupled to thememory, the processor executing the executable instructions to:determine a matrix for K frequency sub-bands, wherein the mobile stationcommunicates with a base station in a multi-carrier cellularcommunication system having a plurality of sub-carriers, wherein theplurality of sub-carriers is divided into the K frequency sub-bands, Kis an integer greater than 1, and the matrix is represented by aprecoding matrix indicator (PMI); determine a first sub-index from aplurality of first sub-indices and a second sub-index for the matrixfrom a plurality of second sub-indices; and transmit, to a base station,the first sub-index and the second sub-index on a periodic physicaluplink control channel (PUCCH) for the K frequency sub-bands, whereinthe first sub-index and the second sub-index are common for the Kfrequency sub-bands, the PMI comprises the first sub-index and thesecond sub-index, the plurality of the first sub-indices are a subset ofall possible first sub-indices, the plurality of second sub-indices area subset of all possible second sub-indices, and a quantity ofcombinations of the plurality of first sub-indices and secondsub-indices is smaller than a quantity of possible combinations of allpossible first sub-indices and second sub-indices.
 9. The apparatusaccording to claim 8, wherein the matrix is one of a plurality ofmatrices, the first sub-index of the plurality of first sub-indicesindicates a first sub-matrix from a first sub-codebook, and the secondsub-index of the plurality second sub-indices indicates a secondsub-matrix from a second sub-codebook, wherein each one of the pluralityof matrices comprises the first sub-matrix from the first sub-codebookand the second sub-matrix from the second sub-codebook.
 10. Theapparatus according to claim 9, wherein each one of the plurality ofmatrices is represented as W^(i), and ${W^{i} = \begin{bmatrix}M_{k_{1}} \\{M_{k_{1}}M_{k_{2}}^{i}}\end{bmatrix}},$ where W^(i) represents the i^(th) matrix, k₁ representsthe first sub-index identifying a first sub-matrix M_(k) ₁ , and k₂represents a second sub-index identifying a second sub-matrix M_(k) ₂^(i).
 11. The apparatus according to claim 9, wherein each one of theplurality of matrices is represented as W^(i), and${W^{i} = \begin{bmatrix}M_{k_{1}} \\{M_{k_{2}}^{i}M_{k_{1}}}\end{bmatrix}},$ where W^(i) represents the i^(th) matrix, k₁ representsthe first sub-index, identifying a first sub-matrix M_(k) ₁ , and k₂represents a second sub-index identifying a second sub-matrix M_(k) ₂^(i).
 12. The apparatus according to claim 9, wherein each one of theplurality of matrices is represented as W^(i), andW ^(i) =M _(k) ₂ ^(i) M _(k) ₁ or W ^(i) =M _(k) ₁ M _(k) ₂ ^(i), whereW^(i) represents the i^(th) matrix, k₁ represents the first sub-indexidentifying a first sub-matrix M_(k) ₁ , and k₂ represents a secondsub-index identifying a second sub-matrix M_(k) ₂ ^(i).
 13. Theapparatus according to claim 9, wherein each one of the plurality ofmatrices is represented as W^(i), and ${W^{i} = \begin{bmatrix}M_{k_{1}} \\{M_{k_{1}}e^{{jd}_{k_{2}}}}\end{bmatrix}},$ where W^(i) represents the i^(th) matrix, k₁ representsa first sub-index identifying a first sub-matrix M_(k) ₁ , k₂ representsa second sub-index identifying d_(k) ₂ , and d_(k) ₂ represents ascalar.
 14. The apparatus according to claim 8, wherein themulti-carrier cellular communication system is a long term evolution(LTE) system or a long term evolution advanced (LTE-A) system.
 15. Anon-transitory computer-readable storage medium comprising instructionswhich, when executed by a computer, cause the computer to carry out thesteps of: determining a matrix for K frequency sub-bands in amulti-carrier cellular communication system, wherein the multi-carriercellular communication system has a plurality of sub-carriers, theplurality of sub-carriers is divided into the K frequency sub-bands, Kis an integer greater than 1, and the matrix is represented by aprecoding matrix indicator (PMI); determining a first sub-index from aplurality of first sub-indices and a second sub-index for the matrixfrom a plurality of second sub-indices; and transmitting, to a basestation, the first sub-index and the second sub-index on a periodicphysical uplink control channel (PUCCH) for the K frequency sub-bands,wherein the first sub-index and the second sub-index are common for theplurality of frequency sub-bands, the PMI comprises the first sub-indexand the second sub-index, the plurality of the first sub-indices are asubset of all possible first sub-indices, the plurality of secondsub-indices are a subset of all possible second sub-indices, and aquantity of combinations of the plurality of first sub-indices andsecond sub-indices is smaller than a quantity of possible combinationsof all possible first sub-indices and second sub-indices.
 16. Thenon-transitory computer-readable storage medium according to claim 15,wherein the matrix is one of a plurality of matrices, the firstsub-index of the plurality of first sub-indices indicates a firstsub-matrix from a first sub-codebook, and the second sub-index of theplurality of first sub-indices indicates a second sub-matrix from asecond sub-codebook, wherein each one of the plurality of matricescomprises the first sub-matrix from the first sub-codebook and thesecond sub-matrix from the second sub-codebook.
 17. The non-transitorycomputer-readable storage medium according to claim 16, wherein each oneof the plurality of matrices is represented as W^(i), and${W^{i} = \begin{bmatrix}M_{k_{1}} \\{M_{k_{1}}M_{k_{2}}^{i}}\end{bmatrix}},$ where W^(i) represents the i^(th) matrix, k₁ representsthe first sub-index identifying a first sub-matrix M_(k) ₁ , and k₂represents a second sub-index identifying a second sub-matrix M_(k) ₂^(i).
 18. The non-transitory computer-readable storage medium accordingto claim 17, wherein each one of the plurality of matrices isrepresented as W^(i), and ${W^{i} = \begin{bmatrix}M_{k_{1}} \\{M_{k_{2}}^{i}M_{k_{1}}}\end{bmatrix}},$ where W^(i) represents the i^(th) matrix, k₁ representsthe first sub-index identifying a first sub-matrix M_(k) ₁ , and k₂represents a second sub-index identifying second sub-matrix M_(k) ₂^(i).
 19. The non-transitory computer-readable storage medium accordingto claim 17, wherein each one of the plurality of matrices isrepresented as W^(i), andW ^(i) =M _(k) ₂ ^(i) M _(k) ₁ or W ^(i) =M _(k) ₁ M _(k) ₂ ^(i), whereW^(i) represents the i^(th) matrix, k₁ represents the first sub-indexidentifying a first sub-matrix M_(k) ₁ , k₂ represents a secondsub-index identifying second sub-matrix M_(k) ₂ ^(i).
 20. Thenon-transitory computer-readable storage medium according to claim 16,wherein each one of the plurality of matrices is represented as W^(i),and ${W^{i} = \begin{bmatrix}M_{k_{1}} \\{M_{k_{1}}e^{{jd}_{k_{2}}}}\end{bmatrix}},$ where W^(i) represents the i^(th) matrix, k₁ representsthe first sub-index identifying a first sub-matrix M_(k) ₁ , k₂represents a second sub-index identifying d_(k) ₂ , and d_(k) ₂represents a scalar.